Method of molding a combustion element of ceramic fibers on a porous support



P 27, 1956 G. WEISS ETAL METHOD OF MOLDING A COMBUSTION ELEMENT OF CERAMIC FIBERS ON A POROUS SUPPORT Original Filed Jan. 17, 1962 5 Sheets-Sheet 1 INVENTORS GEFZHART WEISS KURT W. CORNELY Sept. 7, 1966 G. WEISS ETAL 3,275,497

METHOD OF MOLDING A COMBUSTION ELEMENT OF CERAMIC FIBERS ON A POROUS SUPPORT Original Filed Jan. 17, 1962 5 Sheets-Sheet 2 10! I90 I) FIBER BATH PREPARATION PREPARATION STOCK HOLD TANKS I03 I03c I 103a MOLDING TANK (REACTORS) & 104

DRYER I05 KILN INVENTORS GERHART wuss KURT w. CORNELY FINAL ASSEMBLY Sept. 27, 1965 G. WEISS ETAL METHOD OF MOLDING A COMBUSTION ELEMENT OF CERAMIC FIBERS ON A POROUS SUPPORT Original Filed Jan. 17, 1962 5 Sheets-Sheet 5 INVENTORS GERHART WEISS KURT W. CORNELY United States Patent METHOD OF MOLDING A COMBUSTION ELE- MENT 0F CERAMIC FIBERS ON A POROUS SUPPORT Gerhart Weiss, Flushing, and Kurt W. Cornely, Syosset, N.Y., assignors to American Thermocatalytic Corp., Mineola, N .Y., a corporation of New York Original application Jan. 17, 1962, Ser. No. 166,811, now Patent No. 3,179,156, dated Apr. 20, 1965. Divided and this application Sept. 23, 1964, Ser. No. 403,687

3 Claims. (Cl. 162-103) This application is a division of co-pending application Serial No. 166,811, filed January 17, 1962, now Patent No, 3,179,156.

This patent application relates to an improved radiant heater, and in particular, relates to an improved radiant heater which is a flameless gas combustion device and a principal source of radiant energy.

An important object of this invention is to provide a radiant heater having a chemo-thermal converter or reactor which operates based on the flameless combustion of fuel and wherein the reaction is sustained on or near the outer surface of the reactor causing it to incandesce and thereby produce an output which is mainly radiant in nature.

Another object of this invention is to provide a radiant heater of the above-described type, having improved means for increasing the radiant proportion of the energy output of said combustion device.

Another object of this invention is to provide a radiant heater which is economical to manufacture, which is highly efficient in operation and which achieves substantially complete combustion of the fuel so that almost no carbon monoxide or other partial combustion products are produced.

Another object of this invention is to provide a heater of the combustion type which does not require venting and which is extremely safe in operation.

Another object of this invention is to provide a radiant heater employing a combustion element as the source of energy, which may be supplied with gas from an ordinary source of the gas in liquid phase and which may be supplied With air by aspiration, and which at the same time is highly eflicient in operation and produces substantially complete combustion of the gases.

In accordance with preferred embodiments of the invention, the combustion device is in the form of a refractory cylindrical wall or tube closed at one end and comprising discrete, amorphous, inorganic, ceramic fibers arranged in a homogenous, porous wall structure. Suitable rigidifying means are provided to maintainn the fibers in their tubular wall structure form. The open end of the tube is mounted on a hollow fitting which extends through a metal reflector. Means are provided for causing an air-gas mixture to flow through the fitting into the tube and hence through the pores of the tube wall. These pores are oriented generally in the direction between the inner and outer surfaces of the tube so as to permit gas flow through the wall. The outer surface of the tube wall is adapted to incandesce and remain mechanically thermally stable. The tube structure is adapted to remain mechanically thermally stable when the outer wall surface layer is maintained for prolonged periods of time at high operating temperatures.

The combustion reaction is flameless, and because of the low thermal conductivity of the fibrous structure, there is no flashback of the reaction into the interior of the tube. Furthermore, a high proportion of the thermal output of the tube is in the form of radiant energy. The reflector is positioned to receive and reflect the energy in selected direction.

3,275,497 Patented Sept. 27, 1966 As a further feature of the invention, the outer surface of the combustion tube is surrounded by a spaced, coaxial, cylindrical screen or perforated sheath. This screen is mounted in coupled relation to the combustion tube and reflects a certain amount of the energy impinging thereon as well as permitting a certain amount of the energy to pass beyond it to the reflector.

Briefly stated, some of the radiant energy emitted by the reactor is reflected by the screen back onto the combustion tube so as to increase its surface temperature; some passes through the openings of the screen directly to the reflector, and some is absorbed by the metal screen and then re-emitted outwardly to the reflector at a longer wavelength.

With respect to the exhaust gases of the combustion tube having convection energy, they pass through the holes of the screen and strike the reflector; some of the gases strike the screen and are directed back to the surface of the combustion tube and some of the convective energy of the exhaust gases is absorbed by the metal sheath or screen and causes it to emit radiant energy. A portion of this emission is directed back onto the primary emitter (reactor) thus increasing the latters surface temperature while the balance is emitted in the opposite direction away from the reactor surface (outwardly).

It will also be apparent that some of the convective end radiant energy which strikes the reflector is reflected back to the screen. This is so, since an equithermal (or super thermal) surface acts as a reflector to incident radiation. While the total mechanism is relatively complex, the porous sheath aids significantly in increasing the total radiant output of the entire device, by converting a considerable portion of its convection output into radiant energy. It will be apparent that the effectiveness of the screen will depend in part on its porosity or ratio of solid to open area, and in part upon the radiant coupling between the screen and the combustion tube, i.e., the distance between these two members, the sheath mess (inertia) and a number of other design factors.

Another object of the invention is to provide an improved combustion tube and improved methods of making such a tube, for use in an aspirated system rather than a pump driven system.

A molding bath is made, for example, by dispersing colloidal alumina in water and adding aluminum nitrate in aqueous solution to form a gel. This gel is diluted with water, and fibers are chopped in the dispersion. These fibers are illustratively formed from a melt of alumina and silica and may, for example, be the fibers described in Properties of B & W Kaowool, published in December 1957, by the Babcox & Wilcox Company, 161 E. 42nd St., New York 17, NY. Methyl methacrylate is added to complete the bath.

The inner screen serves as a mandrel and is connected to the suction line of a suitable pump and immersed in the bath for a period of approximately five to twenty seconds, depending upon the thickness of the wall which is to be deposited upon the inner screen or mantle, as well as the vacuum maintained, the viscosity of the bath and other parameters.

After removal of the screen or mandrel from the solution, suction is maintained for a short period of time. The molded tube is then optionally allowed to dry at a temperature of F. to F. for a period of approximately 10 to 60 minutes. During this time, the residual water evaporates, and there is established, together with the fibers and the methyl methacrylate, an undecomposed salt binder.

After drying, the reactor is baked at relatively high temperature, in order to sublime the methyl methacrylate. At the same time, the aluminum nitrate is chemically changed to alumina which coats the fibers. As the result of the subliming of the methyl methacrylate, the tube is highly porous with a minimum of back pressure, this being the desired condition when the aspiration feed system is used.

It is important that the metal salt or other appropriate refractory compound of a metal, which is used, be of a form which will not gel and fill the pores during the heating operation. In addition to aluminum compounds, compounds of zirconium, beryllium, thorium, cobalt, hafnium, titanium, barium, magnesium and strontium may also be used.

The combustion tube, in accordance with this process, exhibits extremely low back pressure and long life and does not require a pressurized feed.

Alternatively, it is possible to modify the combustion tube structure and use it in a pump system, rather than an aspirated system.

Other objects and advantages of this invention will become apparent from the following description, in conjunction with the annexed drawings, in which preferred embodiments of the invention are disclosed.

In the drawings,

FIG. 1 is a perspective view of one embodiment of the heater.

FIG. 2 is an exploded longitudinal section, partly in elevation, of the heater.

FIG. 3 is an exploded longitudinal section of the aspirator.

FIG. 4 is a longitudinal section of the combustion tube and its porous sheath.

FIG. 5 is a block flow diagram of one method of producing the combustion tube.

FIG. 6 is a vertical section of a pressure regulator used in the heater.

MECHANICAL CONSTRUCTION OF HEATER In accordance with one embodiment of the invention, the improved heating system comprises a tank 10 which also serves as a support for the heater, and which is mounted in stand 11. Tank 10 may be any suitable cylindrical container prefilled with a suitable gaseous fuel, such as butane, propane or mixtures in the liquid phase.

Stand 11 may be formed in any suitable way and optionally includes a Wire member bent to provide a bottom leg 12a adapted to rest upon a support, and a further bottom leg 12b coplanar with leg 12a. Said leg 12b is connected at one end to one end of leg 12a and extends diagonally across the base of the stand 11. Further legs 12c extend upstandingly from the other end of legs 12a and 12b and are connected at their upper ends by fittings 13 (only one being shown in FIG. 1) to a collar or band 14 which extends frictionally in any suitable way around container 10. Container 10 is removable from hand 14 for replacement purposes. The fitting 13 may be pivotally attached to band 14 to permit pivoting of container 10 to any desired inclined or verti cal position.

Pressure regulator 15 is fixed to the upper end of container 10 in coaxial relationship therewith. As shown in FIG. 6, pressure regulator 15 has a cylindrical body 42 formed with a plurality of axially successive, communicating bores, as follows: lower end bore 43a of relatively large diameter and internally screw threaded as indicated by the reference number 43b, bore 430 of reduced diameter and internally threaded adjacent its junction with bore 43a, as designated by the reference numeral 43d, bore 43c of increased diameter and upper end bore 43 of still greater diameter and internally screw threaded as indicated by the reference numeral 43g.

An outlet is provided by means of radial bore 15b which is screw threaded as designated by the reference numeral 150 and which communicates at its inner end with further radial bore 15d of reduced diameter which in turn communicates with bore 430 intermediate its ends and above screw thread 43d.

Flat diaphragm 44 is located in bore 43 against the shoulder 43/1 formed at the junction between bores 43f and 43e. Annular diaphragm retaining nut 45, which is externally threaded, is screw-ed into bore 43f and clamps diaphragm 44 against shoulder 43h. Nut 45 has an inturned, transverse, annular flange 45a at its end remote from diaphragm 44.

Knob 46 is externally knurled, as indicated by the reference numeral 460 and is cylindrical and of smaller diameter than bore 43 Knob 46 has a transverse lower end flange 46b of increased diameter which is externally screw threaded at 46c and is thereby screwed into bore 43 The upper end of body 42 is peened over 42a to prevent removal of flange 46b. Knob 46 has an axial bore 46d extending from the free end face of flange 46b toward the other end of knob 46. The upper end of bore 46d is conical, as designated by the reference numeral 46c, and receives a ball 47 which acts as a bearing for tension coil spring 48. This spring 48 is received within bore 46d, and ball 47 extends partly into spring 48.

Diaphragm plate 49 is fixed to the upper face of diaphragm 44 and is received within the opening of nut 45. Plate 49 has an end boss 49a of reduced diameter which extends into the other end of spring 48. As the result of the foregoing, spring 48 exerts a tension upon diaphragm 44, the extent of the tension depending upon the axial position of nut or knob 46 which may be adjusted by screwing it in either direction.

Valve body 50 includes an elongated shank portion 51 which is externally threaded at 51a so that it may be screwed into bore 43 by engagement with threads 43d. Said shank 51 is cylindrical, and is of reduced diameter above thread 51a so as to clear the wall of bore 430. Said shank 51 has an end flange 52 of increased diameter which is located in bore 43a. Flange 52 clamps washer 53 and gasket 54 against the shoulder 54a formed at the junction between bores 43a and 430. A further shank 55 is coaxial with shank 51 and extends from flange 52 in the opposite direction from shank 51.

Valve body 50 has an axial bore 55a in shank 55 and an axial bore 51b of increased diameter in shank 51. The upper end of bore 51b is screw threaded at 510. The junction between bores 55a and 51b is shaped to form a conical valve seat 63a. A generally cylindrical body 56 which is externally screw threaded is screwed into bore 51b preferably to the extreme end of the screw threads 51c thereof. This body 56 has a tubular extension 57 of reduced diameter extending toward flange 52.

The body 56 has a through axial bore 58 in which spring 59 is located. One end of spring 59 carries a coaxial pin 60 which extends toward diaphragm 44 and which is adapted to engage in a hollow 60a of a diaphragm plate 61 mounted upon the lower face of diaphragm 44. The other end of spring 59 carries a coaxial pin 62 which protrudes beyond the end of tubular portion 57 and which carries on its end a valve member 63.

In the screwed down position of knob 46, the force of spring 48 on diaphragm 44 forces valve 63 down closingly on seat 630, preventing gas flow above bore 55a. In the screwed up position of knob 46, shown in FIG. 6, valve 63 is spaced from seat 63a Gas flows through bore 55a, bore 51b, bore 430, bore 43c and bore 15d to bore 150.

In assembly, the lower portion of pressure regulator 15 is screwed onto the top of tank 10 with threads 43b suitably received by an appropriate fitting of tank 10 (not shown) and with shank 55 protruding into the tank to open its internal valve.

Aspirator 17 is adapted to be connected to the pressure regulator outlet bore 15b and includes an elongated body or fitting 18, as well as tubular members 19 and 20. As shown in FIG. 3, body 18 is elongated and has an intermediate portion 18a which may be of any suitable cross sectional shape, such as square. Body 18 has end portions 18b and 18c of reduced diameter and which are externally threaded respectively at 18d and 18a. Optionally, the end portion 180 is of greater diameter than the end portion 18b. Body 18 has a through bore 21 extending axially between the extremities thereof. The rear portion 21a of bore 20 is of increased diameter to provide a generally transverse shoulder 21b at the junction between bore portion 21a and the main portion of bore 21. This shoulder 21b is optionally rounded at its outer portion. Body 18 has a plurality of radial bores 22 extending from the outer periphery thereof to the central bore 21, these bores 22 being located in a common plane and communicating with the main bore portion 21 adjacent the shoulder 21b.

Tube 19 is generally cylindrical and is sized to be slidably received within bore portion 21a. However, the outer diameter of tube 19 is preferably greater than the diameter of the main bore portion 21. Tube 19 has a through bore comprising bore portions 23a and 23b, the front bore portion 23b being of increased diameter. The end portion 19a of tube 19 adjacent the rear end of bore portion 23a is optionally of reduced diameter.

Tube 20 has a generally cylindrical shank sized to be received slidably within bore portion 23b and has a head 20a of increased diameter adapted to abut the end of tube 19. The shank of tube 20 has a bore 24 of approximately the same diameter as bore portion 23a. Head 20a has an axial orifice bore 25 of small diameter which communicates with bore 24. In assembly, the shank of tube 20 is extended into bore 23b with head 20a abutting the end of tube 19, and the two tubes 19 and 20 are then inserted into the bore b of pressure regulator 15, with the head a outermost (in front of tube 19). The tube portion 19a extends frictionally into bore 15d.

The tube portion 180 is then extended over the tubes 19 and 20 with tube 20 received within bore 21a and with the head 20a then conformingly abutting and sealing against shoulder 21b and tube 19. The rear end of the body portion 180 is screwed into bore 15b.

' It will be apparent from the foregoing that when fuel gas is permitted to pass from tank 10 through pressure regulator 15 and hence through bores 23a, 23b, and 24 and orifice of aspirat-or 17, air is aspirated into bore 21 through bores 22 and is there mixed with the fuel gas. The resulting air-gas mixture is discharged through the outer or front end of bore 21.

Reflector 26 may have any suitable shape, which may be generally parabolic. Optionally, it includes rear transverse base portion 27 transverse to the reflector axis and flat and having a central opening 27a. Insulating bushing insert 28 which is internally and externally threaded is screwed onto aspirator element 18b. Metal collar 28a may be screwed over bushing 28. Annular shoulder 28b of collar 28a connects with collar 28a in front of bushing 28 and connects at its inner edge with collar extension 28c. This extends through opening 27a and retaining nut 29 is then screwed onto extension 280 against base 27. Forwardly of nut 29, the base of combustion device 5 may be screwed onto collar extension 280.

Bushing 28 may be made of any suitable heat-insulating material. Optionally, it may be replaced by a ceramic cement insert of similar shape.

The outer portion of reflector 26, designated by the reference numeral 30, may have any desirable shape and is optionally and preferably generally parabolic in shape. The free edge portion of reflector portion 30 is rolled outwardly and back on itself to provide a bead 31.

Any suitable wire guard may have, by way of example, a circular wire frame member 32 which is transverse to the axis of reflector 26, as well as a plurality of longitudinally extending, circumferentially spaced spring legs 33 fixed to frame member 32 and extending in rearwardly relation therefrom toward reflector 26. These legs 33 are optionally paired and are provided with transverse extensions 33a which extend across the face of frame member 32 and are integrally joined at 33b, as shown in the drawing, these details being conventional and not requiring extended description. The free extremities 34 of the fingers 33 may be hook-shaped so as to conform in shape to the head 31. The fingers 33 are normally located inwardly of the periphery of bead 31, but may be sprung outwardly (as shown in FIG. 2) so as to extend the hooks 34 about the outer periphery of bead 31, the spring tension of the bead 33 thereby holding the guard releasably mounted upon bead 31.

The combustion device, shown in detail in FIG. 4, generally includes a disc-like base 35 which has a central forwardly extending cylindrical boss 36. Inner cylindrical screen 37 is mounted upon boss 36 and outer cylindrical screen 38 is mounted on disc 35. Boss 36 has a central extension 39 of reduced diameter, the outer periphery of which is accordingly spaced from screen 37. Screen 37 has a transverse front end wall portion 37a. Screen 38 has a transverse front end wall portion 38a forwardly spaced from wall 37a. A fibrous wall 40, which will be described in greater detail below, is deposited upon the outside of screen 37, including the transverse end wall 37a thereof. The deposited wall includes a portion 40a which fills the space between boss extension 39 and screen 37.

The mounting boss 36 has a through axial bore 41 which extends through the disc portion 35, boss 36 and boss extension 39. The rear portion of bore 41 is internally screw threaded, as indicated by the reference numeral 410..

In the assembly of the device, before mounting the guard on the reflector, the base of the combustion device is screwed onto shank 18b with the threads 18d and 41a interengaged.

MOLDING OF THE COMBUSTION TUBE In accordance with one embodiment of the invention, wherein the aspirator system of feeding the fuel-air mixture is utilized, the combustion tube 40 is molded upon the inner supporting screen 37, after screen 37 has been mounted upon base 35, and before the outer screen 38 has been assembled.

Screen 37 is mounted upon boss 36 and secured thereto by welding 37b. The base 35 is then connected to a source of vacuum, with the base and screen assembly immersed in a molding tank, for the required length of time to vacuum mold the wall material of wall 40 onto the screen.

Illustratively, without limitation thereto, the screen may be 50 x 50 mesh, 0.009 inch diameter wire. The mesh range may be approximately 20 x 20 to 60 x 60. The screen may be made of a suitable rustproof alloy of steel.

As indicated diagrammatically in the flow diagram shown in FIG. 5, fibers are initially prepared in a tank 100, and a bath prepared in tank 101. The fibers and the bath are then mixed in a stock-hold tank 102. Optionally, there may be two such tanks 102 which may be used alternatively, so that one tank can be filled while the other is in use. The mixture of fibers and baths is led from tank 102 to molding tank 103 in which the molding operation takes place.

In this embodiment, the preferred fiber for use in molding the refractory wall 40 is an amorphous melt of alumina A1 0 and silica, SiO One preferred fiber is derived from kaolin and is described in a data sheet published by The Babcock & Wilcox Comp-any, Refractory Division, 161 E. 42nd St., New York 17, N.Y., entitled Properties of B & W Kaowool, dated December 1957, as follows:

Use limit:

Continuous, F 2000 Short periods, "F 2300 Melting point, F 3200 Fiber diameter:

Microns (average) 2.8

Inches (average) 0.00011 Corrosion resistance.-Although the rate of attack is somewhat less for Kaowool than for other ceramic and glass fibers, Kaowool is not recommended where it will be subjected to acids and alkalies for long periods. For very short periods of contact the resistance to cold acids is better than to hot acids, or to hot or cold alkalies.

Density.In bulk form Kaowool may be packed to densities of 3 to 10 lbs. per cu. ft.

In blanket form Kaowool is manufactured in the range of 4 to 8 lbs. per cu. ft. density.

Fiber langth.The length of Kaowool fibers varies. The maximum length of fiber is not over eight inches.

Since the fibers are formed as a melt of alumina and silica at very high temperatures, the fibers do not have any crystalline structure; in other words, they are amorphous. The fibers are produced by treatment of a melt of alumina and silica, at a temperature above that at which crystallization takes place, the treatment to produce the fibers being well known.

One of the features of these fibers is their low thermal conductivity.

In the chemical analysis, the alumina may be increased to approximately 53.3%, with corresponding reduction of silica content.

Another fiber which can be used in accordance with this invention is described in a publication entitled Fibrefrax Ceramic Fiber-Bulk Fiber, published by the Carborundum Company, Niagara Falls, New York, in 1959. The fiber length is up to one and one half inches. The mean diameter of the fibers is two and one half microns. The melting point of the fibers is above 3200 F., and the fibers may be used to 2300 F. The detailed approximate chemical analysis is given in said publication and includes 51.2% A1 47.4% S10 and trace elements. The bulk fiber, at a density of six pounds per cubic foot, has a K factor at a mean temperature of 2000" F. of 2.92.

In general, the preferred fiber in accordance with this invention comprises mainly alumina and silica, each in substantial proportions. The silica is desirable in order to obtain the desired fibrous structure, with the fibers having sufficient mechanical strength.

The alumina is desired in order to impart to the fiber resistance to high temperatures. While the proportions of alumina and silica can be varied from the two examples given above, it is preferred to use commercially available sources of fibers and the above two examples are illustrative of fibers of this general type which are commercially available.

It should be understood that the invention is not limited to the above described fibers, as quartz fibers, vitreous silica fibers, and other fibers can also be used. The fiber is inorganic, amorphous, and resistant to temperatures of 1800 to 2300" F., and preferably has low density, has a diameter of under approximately 10 micronsand has adequate mechanical strength. The fiber is preferably derived from a clay, and is defined as ceramic, it being understood that this term also includes quartz, vitreous silica and other fibers suitable for use which are inorganic, amorphous and resistant to high temperature, and which are amorphous melts of refractory oxides, such as oxides of silicon or alumina or both. Certain appropriate fibers are included among those described in Chapter 8 of Inorganic Fibres by C. Z. Carroll-Proczynski, published in 1958 by National Trade Press Limited, London, England.

In one preferred example, the fibers to be molded are coated and bound with an aqueous solution of aluminum nitrate together with colloidal alumina.

The aluminum nitrate solution has a specific gravity of 1.126, the solution including 14.3% by weight anhydrous aluminum nitrate.

The colloidal alumina is described in a publication entitled Du Pont Baymal Colloidal Alumina, published by E. I. du Pont de Nemours & Company, Wilmington 98, Delaware, in 1961.

Said colloidal alumina is described in this publication as a white, free-flowing powder consisting of clusters of minute fibriles of boehmite (AIOOH) alumina. The surface of the fibriles is modified by adsorbed acetate ions.

The chemical composition and physical properties of the product are set forth on page 10 of the aforesaid Du Pont publication.

In accordance with the preferred example, sufiicient of the aforesaid colloidal alumina is dispersed in water by the procedure set forth on page 31 et seq. of the aforesaid Du Pont publication, to produce a concentrate by weight of the composition of 1.2%.

To three parts of the aforesaid dispersion of the collodial alumina, is added optionally one part by weight of the aforesaid aluminum nitrate solution. The proportions may be varied. The resulting mixture is a highly thixotropic gel. This gel is diluted with water in the ratio of three parts by Weight of water to one part weight of the gel.

It is within the scope of the invention to omit the colloidal alumina and use only the solution of aluminum nitrate. However, in that case, methyl cellulose is first dissolved in water, the methyl cellulose being of 4000 CPS. and made by Dow Chemical Company, Midland, Michigan under the trademark Methocell.

3.4 grams of said methyl cellulose are dissolved in 600 cc. of water held at a temperature of F. to F, agitated for a period of five minutes. To this solution is added 3400 cc. of the aforesaid aluminum nitrate solution, Zogtqaining a final concentration of aluminum nitrate of The advantage of the use of the preferred mix of aluminum nitrate and colloidal alumina is that the methyl cellulose can be eliminated. Also, the presence of the colloidal alumina is desired in the final product.

Optionally, is it possible to use entirely colloidal alumina and omit the aluminum nitrate.

In the preferred example, 3 grams by weight of the aforesaid amorphous refractory fibers are chopped in 500 cc. of the dispersion of aluminum nitrate and colloidal alumina, in tank 102. The fibers are chopped in a high speed shear-type mixer, the mixer being operated first at high speed for approximately 30 seconds and then at low speed for approximately 30 seconds.

In order to complete the bath, preferably in molding tank 103, a weight of methyl methacrylate is added so as to adjust the ratio of fibers to rnethacrylate to selected value. The methyl methacrylate is preferably approximately 20 to 40 mesh. The proportion of methyl methacrylate in the solution is adjusted dependent upon the desired porosity of wall 40, the thickness thereof, the mesh of the deposit screen and other parameters. Illustratively, and without limitation thereto, the porosity may be adjusted by varying the methyl methacrylate:fibcr ratio from 2:1 to 24:1.

ing operation.

to the inner surface, of the alumina coating.

1200 cc. of the aforesaid bath may be maintained in molding tank 103, under agitation except during the mold- A tube 103a, shown diagrammatically in FIG. 5, has an end screw coupling 103k which may be screwed into bore 41 of base 35. The screen 37 may then be immersed vertically in the solution in tank 103, and tube 103a may be connected to any suitable source of vacuum (not shown), such as a pump, so as to suck solution through the screen and into the tube in the direction of arrow 1030. The screen may be maintained in the molding tank for approximately one to approximately ten seconds depending upon the thickness of wall 40 which is desired. Thus, if the wall thickness is to he approximately inch, then the screen or mantle 37 is illustratively kept under vacuum in the solution for a period of about one second. If the wall thickness is to be approximately inch, then the screen is illustratively kept under vacuum in the solution for approximately ten seconds. The vacuum times depend upon various parameters. Optionally, a liquid pump can be used instead of a vacuum pump.

Initially, under the vacuum, some of the slurry passes through the openings of the screen. Some of the slurry is sucked up through bore 41, and some of the slurry is deposited as the wall portion 40a referred to above. Pores of the screen are then fairly well coated with the fibrous slurry, and during the remainder of the vacuum period, very little of the slurry is drawn through the screen. Instead, the slurry is deposited relatively uniformly to form a substantially uniform wall 40.

After the mantle is withdrawn from the solution in tank 103, suction is maintained on it for a period of approximately one minute, at which time the'fitting 1031b is removed. The resulting reactor or combustion device is then dried in any suitable drying apparatus 104, at a temperature of 100 F. to 150 F, for a period of one hour. During this time, residual water in the wall 40 evaporates, and there is established a concentration gradient on the fibrous structure, from the outer to the inner surface thereof, of undecomposed aluminum nitrate and colloidel alumina.

After drying, the reactor is placed in kiln 105 at a tem- "perature of 150 =5. The temperature is increased at the rate of approximately 10 F. to 50 F., preferably 10 to 20, per minute to a temperature of approximately 1100 F. During this time, the methyl methacrylate sublimes leaving an extremely uniform and highly porous wall structure comprising the refractory fibers coated with aluminum oxide formed from the aluminum nitrate as the result of chemical decomposition thereof and also comprising the colloidal alumina.

As an important feature of the invention, there is a concentration gradient on the fibrous structure, from the outer If the firing operation is carried out to temperature less than 1800 F.,

the alumina consists of a mixture of gamma alumina and theta alumina. If the firing operation is carried out at temperature greater than 1800" F. the alumina will consist primarily of alpha alumina.

In ordinary cases, the phase character of the alumina is of little consequence in terms of the operational characteristicsof the combustion device. In certain applications, however, catalytic agents are added to the bath for deposit on the surface of the fibers, and in that case the gamma phase of the alumina is preferable because of its higher surface to mass ratio.

The following table shows the production of both stand- ,ard reactors and thin walled reactors, each set of figures being the average of ten molding operations. The thicker walled reactor, which was denser, had a wall thickness of approximately one-eighth inch and the thin walled reactor, which was more porous, had a wall thickness of one sixtyfourth inch or less. There is no necessary relationship, however, between density and wall thickness.

TABLE More Parameter Denser Porous Reactors Reactors Fiber: Methacrylate 1:12 1:18 Molding Time (seconds) 6-7 1-2 Liquid Passage (co) 250 60- Air Flow Rates (Discharge Side) (l./m.):

1 No load 131 131 2 Molding 16 3 Dry suetion 93 127 Vacuum Readings (mm Hg) 1 N 0 load 100 2 Molding 560 3 Dry suction I 183 Elementary Parameters:

1 Wt. Fibers Deposited (g.) 0.75 0.10 2 Wt. Methacrylate Deposited (g.) 7.6 0.20 3 Percent Fiber Deposited of total Fiber Content 25 3. 33 4 Percent Methacrylete Deposited of total Methacrylate 211 0. 37 5 Fiber/Methacrylate Ratio (wt./wt.) 1:10. 8 1:2 6 Slant Gauge Pressure at 30 l./m. flow (mm. Hg): (1) Fibers and Mandrel back pressure 0.095 0. 025 (2) Mandrel back pressure 0.019 0.019 (3) Fibers only back pressure 0.076 0. 006

1 Could not be determined due to too rapid a deposition rate.

In the above table, the initial figures give the starting ratio by weight of fiber to methyl methacrylate, the molding time and the amount of liquid which passed through the base and was discharged.

The part of the table relating to air fiow rates shows the rate of fiow of air through the screen and into the suction tube, prior to molding, during molding and during the one minute of suction after molding was concluded.

The vacuum readings are given under similar conditions.

The final part of the table gives the final composition of the tube. Thus, in the case of the denser walled reactor, the ratio by weight of fiber to methyl methacrylate was one to 10.8 while in the case of the more porous reactors, the corresponding ratio was one to two.

In accordance with this invention, it is important to use in the bath a metal salt, such as a chloride or nitrate, which will break down into the oxide during the firing operation, or a colloidal metal oxide, or a combination of both, which will have refractory properties and which will coat the fibers. The coating of alumina on the fibers increases their strength and refractory properties. At the same time, the formation of a gel which would plug the pores is undesirable.

In this embodiment, while other fillers than methyl methacrylate can be used, this is preferred because of the uniformity of results obtained with the use thereof. In particular, this material sublimes and leaves no carbon deposit. The filler should be relatively low in cost, safe to use and usable with an aqueous bath, if the filler granules should be larger than the molding screen openings.

Camphor or menthol are other examples of suitable fillers.

The filler should sublime or de-compose in such a way as to leave no residue. Thus, if a carbon residue were left, after the heating step, such residue could only be removed by heating to at least 1,400 R, which could damage the combustion device.

The provision of wall portion 40a is important, because in operation screen 37 expand under the influence of heat while the wall 40 does not expand to the same extent. This would, after a number of cycles of operation, tend to cause the development of a leakage path between the outer surface of the screen and the inner surface of the wall 40, down to the base portion 35. It is necessary to permit freedom of movement of the fibrous wall 40 at the base of the screen, since if the fibrous coating were cemented to the screen, the fibrous coating 40 would crack at some point along its surface. The wall portion 40a serves as a flexible seal offering greater resistance to gas flow than the main portion of wall 40, and prevents the escape of gas except through the wall 40 even after repeated expansions of screen 37.

The fibers anchor on the screen and form a mechanically stable wall, but with suflicient flexibility and low enough K factor to accommodate expansion and contraction of screen 37 during operation of the combustion device.

After the final formation of wall 40, the P eviously mentioned screen 38 may be placed over the base 35 and secured in place by spot welds 38b. Optionally and preferably, screen 38 has a plurality of holes 38c, as illustratively shown, for reception of a match or other means for ignition.

OPERATION After assembly of the device, knob 46 may be turned so as to admit gas to the aspirator and hence cause the admission of the fuel gas/ air mixture into the interior of screen 37. The mixture then flows through the wall 40. Upon insertion of a lighted match through a hole 38c, ignition occurs.

The wall 40 serves as a refractory Wall comprising discrete, amorphous, inorganic fibers arranged in a homogenous, porous wall structure having opposed inner and outer surfaces, the fibers being intermeshed and supported upon screen 37. The fibers are also coated and bonded to each other with a refractory aluminum compound. The porosity and thermal conductivity of wall 40 is such, in conjunction with the rate of fiow of gas/air mixture therethrough, that the fibers on the outer surface layer of wall 40 incandesce and combustion reaction is flameless and complete. The Wall surface remains mechanically and thermally stable when the wall surface layer is maintained for prolonged period of time at high operating temperature.

A high proportion of the energy emitted as the result of the combustion reaction is radiant energy. However, the exhaust gases emitted from wall 40 also carry a certain amount of convection energy. As an important feature of the invention, a good deal of the convection energy of the exhaust gases is converted into radiant energy.

Some of the radiant energy emitted by wall 40 strikes the Wall of screen 38 and is reflected thereby back onto the outer surface of wall 40, thus raising the temperature of wall 40. Some of the radiant energy emitted by wall 40 strikes the screen 38 and is absorbed thereby and screen 38 re-radiates energy at a lower frequency. Some of the radiant energy emitted by wall 40 passes through the openings of screen 38. Some of the exhaust gases emit-ted by wall 40 strike screen 38, and energy is absorbed by screen 38 and converted into radiant energy, and then re-radiated in either direction, thus reducing enthalpy losses of the exhaust gases. Some of the exhaust gases pass through the screen, as losses. However, the parameters such as open area of screen 38, diameter of the wire thereof (or diameter of the closed area and thickness of the element if it is a sheath) and radial distance between wall 40 and screen 38 are adjusted so as to minimize such losses.

The space between wall 40 and screen 38, designated by the reference numeral 9, serves as a partially resonant chamber for conversion of a good deal of the convection heat energy of the exhaust gases into radiant energy. The degree of resonance will depend upon a large number of factors, including the dimensions of chamber 9 and other parameters mentioned in the previous paragraph. The radiant energy and exhaust gases which leave screen 38 strike reflector 26 and are reflected thereby. It will be apparent that in a similar manner, there is a certain amount of coupling between reflector 26 and screen 38, but this coupling is less than the coupling between the outer surface'of wall 40 and said screen 38.

As the result of the absorption and re-radiation of energy by screen 38, it also incandesoes.

While it has been known to interpose a screen over a source of flame, said screen thereby absorbing a certain amount of the energy of the gases and re-emitting it as radiant energy and thereby incandescing, in such case there is no resonant coupling between the flame source and the screen since the flame source is not itself incandescent. In contrast, in the present application, the use of the screen 38 is highly advantageous because it increases the radiant output of the device significantly, particularly since much of the energy incident upon the screen 38 is already radiant.

While we have disclosed a preferred embodiment of the invention, and have indicated various changes, omissions and additions which may be made therein, it will be apparent that various other changes, omissions and additions may be made in the invention Without departing from the scope and spirit thereof.

What is claimed is:

1. Method of molding a combustion element comprising mixing discrete, amorphous, inorganic, ceramic fibers in a liquid bath containing a refractory metal compound which upon heating converts to the oxide of said metal, adding to the bath containing said fibers granules of a filler which is adapted to separate from said fibers on heating without leaving substantial residue, the filler:fiber ratio being from 2:1 to 24: 1, immersing a porous support in said bath, passing bath through said support in a manner so as to deposit on one side of said support material from said bath of greater size than the size of the pores thereof, and heating the support and deposited material sufiiciently to convert said refractory metal compound to the oxide of said metal, to evaporate residual bath liquid therefrom and to remove said filler.

2. Method of molding a combustion element comprising forming a bath of an aqueous dispersion of colloidal alumina together with an aqueous solution of aluminum nitrate in proportions to form a gel, diluting the gel with suflicient water so that a molded water-laid product may be formed, mixing discrete, amorphous, inorganic, ceramic fibers in said bath, adding methyl methacrylate to said bath containing said fibers, the methyl methacrylate:fiber ratio being from 2:1 to 24:1, immersing a porous support in said bath, said support having at least two sides, exerting suction on a first side of said support to deposit solids of said bath on the second side of said support to form a wall thereon, and heating said Wall to sufiicient temperature to convert said aluminum nitrate to aluminum oxide, to cause residual water therein to evaporate and to cause said methyl methacrylate to sublime.

3. Method of molding a combustion element comprising forming an aqueous solution containing aluminum salt, mixing discrete, amorphous, inorganic alumina-silica fibers in said solution, adding methyl methacrylate granules to said solution containing said fibers, the methyl methacrylatezfiber ratio being from 2:1 to 24:1, immersing a porous support in said solution, said support having an interior side and an exterior side, applying suction to said interior side of said support to form a wall on said exterior side thereof, and heating said wall to sutficient temperature to cause residual water therein to evaporate and to cause said methyl methacrylate to sublime.

References Cited by the Examiner UNITED STATES PATENTS 1,855,497 4/1932 Stuart 162l05 2,593,507 4/1952 Wainer 264-63 3,100,734- 8/1963 Rex et al. 162152 3,125,618 3/1964 Levinson 264-63 DONALL H. SYLVESTER, Primary Examiner.

S. L. BASHORE, Assistant Examiner. 

1. METHOD OF MOLDING A COMBUSTION ELEMENT COMPRISING MIXING DISCRETE, AMORPHOUS INORGANIC CERAMIC FIBERS IN A LIQUID BATH CONTAINING A REFRACTORY METAL COMPOUND WHICH UPON HEATING CONVERTS TO THE OXIDE OF SAID METAL, ADDING TO THE BATH CONTAINING SAID FIBERS GRANULES OF A FILLER WHICH IS ADAPTED TO SEPARATE FROM SAID FIBERS ON HEATING WITHOUT LEAVING SUBSTANTIAL RESIDUE, THE FILLER:FIBER RATIO BEING FROM 2:1 TO 24:1, IMMERSING A POROUS SUPPORT IN SAID BATH, PASSING BATH THROUGH SAID SUPPORT IN A MANNER SO AS TO DEPOSIT ON ONE SIDE OF SAID SUPPORT MATERIAL FROM SAID BATH OF GREATER SIZE THAN THE SIZE OF THE PORES THEREOF, AND HEATING THE SUPPORT AND DEPOSITED MATERIAL SUFFICIENTLY TO CONVERT SAID REFRACTORY METAL COMPOUND TO THE OXIDE OF SAID METAL, TO EVAPORATE RESIDUAL BATH LIQUID THEREFROM AND TO REMOVE SAID FILLER. 